Got my hd 6xx a few days ago and I'm almost disappointed to be honest. I plugged them into my Nexus 6p expecting them to live up to the hype. Obviously they aren't going to be at their full potential just plugged into a phone but still, I had the volume all the way up on my phone, and the 6xx was surprisingly quiet. Usually I don't need to go higher than 14/15 volume levels for it to be almost too loud but these headphones we're still very quiet at full volume compared to the almost deafening sound at full volume from nearly any other headphones I've owned.
If anyone knows why they're quiet or anything to help, please do. I can't even really try to compare the quality of the sound if it isn't loud enough for me to hear.

Okay thanks, I'm not too educated on all of these things so that helps. I just plugged them into my laptop and they are certainly capable of louder volumes. They sound incredibly clear, the bass is strong and I feel like I can hear every single instrument. And this isn't even with any amp or anything. So that little rant was unjustified I guess. I didn't give them a proper chance. Is there anything I can do to make them louder and sound better on my phone? or is it just not really meant for that type of casual listening?

Not really meant for casual listening. HD 6x0 lineup is generally considered to need an amplifier. You can pick up a Magni 3 if you want a desktop unit. Otherwise a dacport slim right from massdrop would work pretty well for your laptop. You can look into portable amp/dacs as well - can't name any off the top of my head. There are a bunch from fiio.
My HTC One M9 has a decent headphone output and can make my 6XXs pretty loud fwiw(actually unlistenably so). Guess the Nexus 6p has a pathetic headphone output.

Hey thanks for the input, my nexus 6p has pretty good headphone output from what I've experienced, that's why I was so shocked that it was quiet. On my AKG K553 Pro my phone provides more than enough power to make it loud, so I was concerned when the highly praised hd 6xxs lacked in volume. I don't know much about amps/dacs and what to look for in specs for them so I'll have to do some research I guess.

I have the K553 Pro as well and those are reaaaally easy to drive. Some of the easiest in fact. I have IEMs/earbuds that take more power than the K553 Pro.
Good luck with research! There's plenty of info out there. Also maybe check our reddit.com/r/headphones for some help as well.

You need a better amp/dac then what your phone has. If you need portability then something like a dragonfly. The 6xx is a little harder to drive them other 200 dollar headphones and I am not sure how well the dragonfly could handle them. A desktop solution like a modi 2 and magni 3 will be plenty good enough.

Just jumping in to try to make things a little clearer:
Every headphone has a certain level of impedance. I won't go all into the math (because I don't even completely understand it all), and this is usually overlooked by people getting into these sorts of headphones. Impedance is measured both in the headphone and in whatever is outputting to the headphone, in your case, this would be the phone or laptop. Impedance is, effectively, the electrical opposition, so, higher impedance headphones will need more power to perform their best in volume, clarity and dynamics.
A general rule is that the output impedance should be less than 1/8th the headphone impedance to minimize distortion. With that said, volume is generally measured through through knowing two things: how many decibles (dB) goes into a milliwatt (mW) (dB/mW) and how many mW is outputted at different impedance. So, the trick is that your output source has to be able to put at least enough power to be able to effectively drive the headphone at their impedance level. Typically, it's easier to put more power into lower impedance than into higher impedance.
So, the HD6XX has an impedance of 300 ohms, whereas the AKG K533 Pro has an impedance of 32 ohms.
What I would guess is that your phone just can't supply enough mW into 300 ohms of impedance, but supplies an adequate amount of mW into 32 ohms, which would explain the difference in volume that you're experiencing.
There's a lot more technical data that goes into it, but hopefully this gives you some idea as to where to start. Cheers!

It's sad that you were expecting these to be a good phone headphone. Not at all. This is a medium-high impedence headphone. I would've done a bit more research before dropping $200 on something that wouldn't work how I wanted it to.